A laser consists of a pumped gain medium placed within an optical resonator. The pumped gain medium provides optical amplification, and the optical resonator provides optical feedback, such that light can circulate within the optical resonator and be repeatedly amplified by the gain medium. Frequently the optical resonator is referred to as the laser cavity. Various pumps are known, such as optical pumps and electrical pumps. The light wavelength need not be in the visible part of the electromagnetic spectrum. If the round trip loss within the optical resonator is less than the round trip gain provided by the gain element, the optical power increases on each round trip around the cavity. Since the amplification provided by the gain element decreases as the circulating optical power increases, the steady state circulating power is the power required to make the round trip gain equal to the round trip loss. One of the elements within the optical resonator acts as the output coupler, whereby a certain fraction of the circulating power is emitted from the optical resonator, and constitutes the laser output. A partially transmitting mirror is a typical output coupler.
An external cavity semiconductor laser is one type of laser. As light makes a round trip within an external cavity semiconductor laser, light is emitted from a pumped semiconductor gain medium, passes through various optical elements, and impinges on the gain medium as a return beam. Typically, multiple semiconductor layers are epitaxially grown on a semiconductor substrate to form a semiconductor gain medium, and the gain medium waveguide is formed by lithographic processing of some or all of the epitaxially grown layers. The resulting waveguide is contiguous with the substrate. That is, the waveguide is either in direct contact with the substrate, or there are one or more intervening solid layers between the waveguide and the substrate. The epitaxially grown layers can have various compositions, which may or may not be the same as the composition of the substrate.
An optical beam emitted from a single-mode optical waveguide has an amplitude and phase profile determined by the waveguide, which is referred to as the mode profile. The amplitude and phase profile of the return beam is generally not exactly the same as that of the mode profile, and in such cases, not all of the return beam power is launched (i.e. coupled) into the gain medium waveguide. For example, if a certain power Pb impinges on the waveguide endface, only some lesser amount of power P0 is actually launched into the waveguide. The coupling efficiency η=P0/Pb depends on how close the return beam amplitude and phase profile is to the mode profile.
The laser emission wavelength is the wavelength at which the net gain (i.e. gain −loss) is maximal. If the gain medium provides amplification over a wide wavelength range and the spectral dependence of the loss is dominant (i.e. the difference between minimum loss and maximum loss at different wavelengths is large compared to the gain), then the laser emission wavelength will closely approximate the wavelength at which the round trip loss in the resonator is minimized. For example, if the wavelength of minimum loss is λ0, and the laser emission wavelength is λ1, the wavelengths λ0 and λ1 will differ if the wavelength dependence of the gain is strong enough that the round trip net gain is maximized at a wavelength which differs only slightly from the wavelength of minimum loss. Thus, the most common way to make a tunable laser is to insert one or more optical elements within the laser cavity to create a tunable intracavity bandpass filter. Since a tunable bandpass filter has lower loss for a narrow range of optical wavelengths centered about a tunable center wavelength λc, and higher loss for wavelengths outside this range, such a filter will tune the laser emission wavelength. In this case, the difference between λ0 and λ1 will be no larger than the filter bandwidth.
The use of an etalon to provide an intracavity bandpass filter for laser tuning is known [e.g. Zorabedian et al., Optics Letters 13(10) p826 1988; U.S. Pat. No. 5,949,801 Tayebati; U.S. Pat. No. 6,301,274 Tayebati et al]. An etalon comprises two nominally parallel, partially transmitting mirrors arranged to form an optical resonator. It is known that etalon mirrors need not be exactly parallel to form an optical resonator. Transmission through an etalon is generally low, except for a series of peaks, which are approximately equally spaced at an interval known as the free spectral range, as seen in FIG. 2a. Since the center wavelength of an etalon transmission peak can be varied by changing the optical distance between the etalon mirrors, an etalon in transmission is known to be a suitable laser tuning element. The optical distance dopt between two points a and b is given by       d    opt    =            ∫      a      b        ⁢                  n        ⁡                  (          x          )                    ⁢                           ⁢              ⅆ        x            where n(x) is the position-dependent index of refraction.
Naturally, it is necessary for the free spectral range to be substantially larger than the desired tuning range, to ensure that only one of the etalon transmission peaks is within the desired tuning range. The bandwidth of the transmission peaks is also an important parameter for laser tuning, since bandwidth determines the loss seen by the modes adjacent to the lasing mode, which in turn determines the side mode suppression ratio (SMSR). Both the bandwidth and free spectral range of an etalon can be varied according to known design principles.
Reflection from an etalon is generally high, except for a series of valleys of low reflectivity, which are approximately equally spaced at the free spectral range, as seen in FIG. 2b. As seen in FIGS. 2a and 2b, the etalon reflectivity is high where the transmissivity is low, and vice versa. Because the reflection spectrum of an etalon does not provide a narrow bandpass filter, an etalon would not be expected to act as a tuning element in reflection. See, for example, Siegman, Lasers, University Science Books, Mill Valley Calif. 1986, pp 423-427, which describes the use of a reflective etalon as an output coupler for a high power laser. In this case, the etalon is acting as a mirror, not as a tuning element.